1. Field of the Invention
The present invention relates to wireless communication, and more specifically to a method and a device for transmitting midamble signals in a wireless communication system.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4th generation) mobile communication standard following the IMT-2000. It was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
Technology for supporting reliable and high-speed data service may include multiple input multiple output (MIMO). MIMO technology can be used to improve the efficiency of data transmission and reception using multiple transmission antennas and multiple reception antennas. MIMO technology includes spatial multiplexing, transmit diversity, beam-forming and the like. An MIMO channel matrix according to the number of reception antennas and the number of transmission antennas can be decomposed into a number of independent channels. Each of the independent channels is called a layer or stream. The number of layers is called a rank.
A pilot may be transmitted from a base station to a terminal through a downlink. The pilot may be referred to as a reference signal in accordance with a wireless communication system. Channel estimation may be performed or a channel quality indicator (CQI) may be measured using the pilot. The CQI may include a signal-to-interference noise ratio (SINR) and frequency offset estimation. In order to optimize performance of a system in different transmission environments, an IEEE 802.16m system provides a common pilot structure and a dedicated pilot structure. The common pilot structure and the dedicated pilot structure may be distinguished from each other in accordance with used resource. The common pilot may be used by all terminals and may be precoded by the same method as a method by which a data subcarrier in the same physical resource unit (PRU) is precoded. The dedicated pilot may be used for allotting localized and distributed resources. The dedicated pilot may be used by a terminal to which specific resource is allocated. Therefore, the dedicated pilot may be precoded or beamformed by the same method as a method by which the data subcarrier is precoded or beamformed. In the pilot structure, up to eight transmission streams may be defined. A unified pilot structure may be provided in accordance with the common pilot and the dedicated pilot.
On the other hand, midamble is signals transmitted by the base station so that the terminal may directly measure channel states. When the base station transmits the signals using a multiple-input and multiple-output (MIMO) technology through a plurality of antennas, different signals are transmitted to the antennas, respectively, in different positions in a resource region and the terminal receives the midamble to measure the channel states of the antennas of the base station and to estimate the channel state of a serving cell or the interference level of a neighbor cell. The base station may receive the channel sates estimated by the terminal to adaptively schedule resource.
When the plurality of antennas exist, a method for efficiently transmitting the midamble signals is required.